The global increase in population and industrialization has required efficient treatment methods and recycling methods of waste water containing various chemical substances. On this account, various water treatment methods have been developed and used. Examples of such a method include a solid phase adsorption method using activated carbon or an ion-exchange material, a separation method using a hollow fiber membrane, an electrochemical method employing electrodialysis or other techniques, a coagulation-sedimentation method, a coprecipitation method, and an activated sludge method. These methods are properly used depending on the quality or amount of waste water to be treated. Among them, the solid phase adsorption method is excellent in environmental load and operability.
The solid phase adsorption method employs adsorbents having a wide variety of shapes. Among them, an ion-exchange fiber, which has a larger specific surface area than that of ion-exchange resin beads, typically achieves excellent adsorption speed. The ion-exchange fiber can also be processed into an arbitrary form, for example, non-woven fabric, woven fabric, thread, and chenille yarn. Focusing on such characteristics has led to the development of various fibrous adsorbents.
Patent Document 1, for example, discloses a method for removing and adsorbing copper ions in waste water by treating waste water exhausted from a copper pyrophosphate plating process with a weak basic anion-exchange fiber having a polyethylenepolyamino group. However, the method intends to remove copper ions alone, and thus there is a demand for techniques capable of removing and adsorbing other various chemical substances present in water.
An additional ion-exchange fiber is, for example, a fiber containing an amino group. The fiber is obtained by treating an acrylic fiber with an amine compound to introduce a cross-linked structure and an amino group at the same time (see Patent Document 2). Patent Document 2 describes that the acrylic fiber may be, in addition to a typical acrylic fiber, a modacrylic fiber that is obtained by copolymerization with, for example, vinyl chloride and has high chemical resistance, but describes no modacrylic fiber in Examples. Using the modacrylic fiber in a practical reaction causes dehydrohalogenation as a side reaction to reduce the fiber strength, and thus the method fails to produce an ion-exchange fiber capable of withstanding practical processing and use.
Another ion-exchange fiber is an ion-exchange fiber obtained by introducing a cation-exchange group or an anion-exchange group to a fiber that is prepared by polymer-blending a modacrylic polymer and an epoxy group-containing polymer (see Patent Document 3). The method disclosed in Patent Document 3 uses a modacrylic fiber and can produce an ion-exchange fiber. However, the method introduces a substituent to an epoxy group alone, and accordingly has a limitation to the amount of ion-exchangeable substituent capable of being introduced. Thus, there is a demand for improvement in the ion-exchange capacity.
In addition to the ion-exchange fibers, various ion-exchange fibers are disclosed, including an anion-exchange resin produced by reacting an aromatic cross-linked copolymer having a haloalkyl group with an amine in the presence of predetermined amounts of water and an inorganic salt (see Patent Document 4), an ion-exchange fiber produced by introducing primary to quaternary amines to polyvinyl alcohol (see Patent Document 5), a polymer containing a predetermined amount of a potassium carboxylate group and having a cross-linked structure (see Patent Document 6), a polymer cross-linked through a particular structure between chitosan molecules (see Patent Document 7), and a cation exchange fiber produced by introducing a predetermined amount of a carboxyl group to an acrylic fiber (Patent Document 8). Each of the resins and the fibers still has room for improvement in the adsorption and removal performance of chemical substances.
In a nuclear power plant, especially in the cooling system or the exhaust system of a nuclear reactor, a nuclear fuel rod having a breakage such as a pinhole exhausts fission products such as 129I and 131I. Among them, 129I has a long half-life of 107 years but is exhausted in a small amount and has a low energy. On the other hand, 131I has a short half-life of 8 days but is exhausted in a large amount and has a high energy. Accordingly, the most harmful fission product from the waste water and exhaust systems of a nuclear reactor is 131I, which is a subject to be measured and evaluated in a nuclear facility.
It takes a long period of time to carry spent nuclear fuel from a nuclear facility to a spent nuclear fuel reprocessing plant, at which the amount of 131I having a short half-life is small but the amount of 129I having a long half-life is large. Typical chemical forms of radioactive iodine exhausted from a nuclear facility are considered to be three forms of iodine (I2), hydroiodic acid (HI), and methyl iodide (CH3I). Among them, the nonionic methyl iodide is most difficult to be removed.
Existing methods of removing radioactive iodine exhausted from nuclear facilities are as follows:
(1) A large amount of alkali-impregnated activated carbon impregnated with potassium iodide (KI) is used for isotope exchange of 131I as the radioactive iodine for nonradioactive iodine to thus collect the radioactive iodine.
(2) A gas or liquid containing iodine is brought into contact with impregnated activated carbon impregnated with triethylenediamine (TEDA) or with a strong base anion exchanger to react a tertiary amino group with methyl iodide to thus remove iodine.
(3) A gas or liquid containing iodine is brought into contact with silver zeolite to collect the iodine as silver iodide.
Among them, to remove methyl iodide, the removal techniques (2) and (3) are applied. Hydroiodic acid, which is acidic, can be removed with alkali-impregnated activated carbon or a strong base anion exchanger. In addition, the method of removing molecular iodine (I2) is exemplified by a method of absorption to KI and a method of employing a polymer material produced by graft polymerization of polyvinylpyrrolidone.
However, the above methods employing impregnated activated carbon impregnated with potassium iodide or TEDA require the activated carbon in a large amount, and this increases the cost and concurrently causes a disposal problem of the activated carbon used. The method employing silver zeolite requires expensive silver zeolite. It also requires dehydration, heating at 150° C., and other steps to complicate the process, and fails to achieve a satisfactory removal ratio of radioactive iodine.
There is a much larger problem. The method for removing ionic substances such as iodine (I2) and hydroiodic acid is quite different from that for removing nonionic substances such as methyl iodide. In order to completely remove radioactive iodine from a system containing such ionic substances and nonionic substances, both methods are required to be combined and this complicates the removal method.
As described above, there is a demand for an ion-exchange fiber capable of efficiently removing and adsorbing not only various chemical substances in waste water but also chemical substances such as radioactive iodine compounds and for a method for removing and adsorbing chemical substances in water.